Anaerobic digestion is used to convert a variety of organic materials to gas and soluble constituents. Anaerobic digestion is commonly used to convert a fraction of sewage sludge, animal manure, and other putrid organic slurries containing substantial quantities of pathogenic bacteria, viruses, fungi, and parasites to methane gas and soluble products. Conversion of sewage sludge, or other waste solids to gas and soluble constituents lowers the cost of solids disposal by reducing the mass of solids requiring disposal. Examples of process that are useful for converting organic materials to gas and soluble constituents include the retained biomass processes described in U.S. Pat. Nos. 5,015,384 and 5,670,047 by the inventor of the subject application. The '384 patent describes a retained biomass process similar to the one illustrated in FIG. 1. In such a process, an influent slurry 1 is delivered at a rate Q to anaerobic reactor 8. Solids, comprising partially digested solids and anaerobic bacteria, and liquid and soluble products of digestion are delivered via line 2 to a separator 9 where the solids are separated from the liquid and soluble products of digestion. The '384 patent describes that the solids are either returned to anaerobic
Retained biomass systems substantially improve the economics of anaerobic digestion. The digestor size can be significantly reduced while improving the percent solids converted to gas and soluble products. In a retained biomass system solids must be wasted from the system on a periodic basis to avoid their undesirable build up. Referring to FIG. 1 and 2, solids can be wasted from separator 9 along line 5 or from the thickened return solids stream 3 shown in FIG. 1. Solids can also be wasted from the digestor 8 or from line 2 as shown in FIG. 2. Typically, it is more economical to waste the concentrated solids from the thickened solids stream 3 or directly from the separator 9 shown in FIG. 1, as compared to wasting the less concentrated digestor solids as shown in FIG. 2.
In addition to the volume and mass of residual waste solids, the cost of disposing waste solids is also influenced by the quality of solids requiring disposal. Solids, which have lower concentrations of bacteria and viral pathogens, can be disposed at less cost than solids containing higher concentrations of pathogenic organisms. Current regulations (EPA--40 CFR Part 503) define Class A or "exceptional quality" biosolids as those waste solids containing a reduced quantity of pathogenic bacteria such as salmonella, and fecal coliform. Class A biosolids can be produced through a variety of means including:
1. Disinfection by holding the waste solids (biosolids) at an elevated temperature (above 50.degree. C.) for a prolonged period of time. Shorter detention times are achieved at higher temperatures. Pasteurization is achieved by retaining the solids or slurry for 30 minutes or more at 70.degree. C. or higher. PA1 2. Raising the pH above 12 for 72 hours at an elevated temperature, or drying to 50% solids concentration. PA1 3. In vessel or aerated composting for three days at 55.degree. C. PA1 4. Windrow composting at 55.degree. C. for 15 days PA1 5. Heat drying at 80.degree. C. PA1 6. Heat treatment of liquid biosolids to 180.degree. C. for 30 minutes. PA1 7. Aerobic thermophilic digestion at 55.degree. C. for 10 days PA1 8. Beta ray irradiation PA1 9. Gamma ray irradiation PA1 1. The energy or chemical cost required to elevate and maintain the temperature of the waste biosolids; and PA1 2. The reactor size required to hold the waste solids for the required period of time. PA1 1. Plug flow anaerobic thermophillic digestion or simulated plug flow digestion through a series of completely mixed thermophiluic digesters. In this type of process the entire influent flow is heated to thermophlllic temperatures. Heat losses from the digestors can be significant, since the volume to surface area required for multiple digestors is low. High effluent organic acids and ammonia concentrations, and the poor dewaterability of the residual solids are significant disadvantages of anaerobic thermophillic digestion or simulated plug flow digestion. PA1 2. Batch pasteurization of the entire influent flow prior to anaerobic digestion is a commonly used practice. However, pasteurization of the entire influent flow prior to anaerobic digestion is expensive since the entire viscous flow must be pasteurized. The procedure requires large quantities of energy for transporting and heating the viscous influent slurry. PA1 3. Thermophillic anaerobic digestion followed by mesophillic anaerobic digestion (Dague, et al #5,746,919) has been described as useful to achieve Class A biosolids. However, in order to meet the strict requirements of EPA--40 CFR Part 503, in such a process the thermophillic digestor should operate in a plug flow mode to prevent short-circuiting, i.e. passage of a portion of the biosolids without being exposed to the treatment conditions for a sufficient period of time to effect the desired results. Again, with this type of process, the entire flow of biosolid stream must be heated to thermophillic temperatures and maintained at those temperatures for a considerable period of time, generally exceeding 7 days in the thermophillic reactor and 12 days in the mesophillic digestor. Such long retention times and large volumes contribute significantly to the overall cost of the process. In addition, two biomasses, mesophillic and thermophillic, are grown, thus reducing the mass converted to gas while increasing the mass to be disposed. PA1 4. Thermophillic aerobic digestion followed by mesophillic anaerobic digestion also provides a process to achieve Class A biosolids. However, in order to meet the strict requirements of EPA--40 CFR Part 503, in this process the hermophillic aerobic digestor is generally operated in the plug flow mode or as a sequencing batch reactor to prevent short-circuiting. Again the entire flow must be heated to thermophillic temperatures and maintained at those temperatures for a considerable period of time, generally exceeding 5 days in the aerobic reactor, and 12 days in the anaerobic digestor. Costs associated with the heating, transporting and storing the process stream can be significant. In addition, a significant portion of the methane gas generating potential is lost in the aerobic portion of such process. PA1 5. U.S. Pat. No. 5,888,453 describes a continuous flow pasteurization process for sewage sludge. The process described in the '453 patent involves raising the temperature of the sludge by passing the sludge through at least one heat exchanger supplied with a heating fluid. The sludge at the elevated temperature is delivered to a detention tank where it is held above a predetermined temperature for a predetermined period of time to effect pasteurization. The '453 patent describes that the pasteurized sludge can be fed to a second anaerobic reactor for further digestion, followed by delivery to a dewatering system.
As can be observed from the list above, the waste biosolids must have the pH or the temperature elevated for prolong periods of time to achieve the desired pathogen reduction. The cost of achieving the desired pathogen reductions according to the above methods is directly related to:
The quantity of chemicals used, the energy required, and holding vessel size, are each dependent on the quantity of solids that must be processed to reduce the pathogen concentration.
Specific methods of producing anaerobically digested "Class A" biosolids include the following processes:
There continues to be a need for an energy efficient anaerobic digestion process capable of effectively and efficiently digesting organic substrates and producing residual solids that contain reduced quantities of pathogens so as to be classified as Class A or "exceptional biosolids" by the EPA.